video processing machines for simulating three dimensional movement of a live video picture are known. Hitherto, the controls for such machines have included switching devices for selecting operations and movement detectors, such as levers and tracker balls, for defining the temporal nature of the selected effect. These controls provide a means by which a user of the machine can among other things achieve the effect of apparent three dimensional movement of for example live video pictures on a display screen. However, such controls are limited in that multiple actuations of different levers and switches together with movement of the tracker ball are required in order to achieve the desired effect. Thus, efficient use of the machine requires a high level of experience and dexterity on the part of the user and, whilst highly satisfactory effects are achievable using these controls, an undue amount of time can be spent by the user in achieving desired results.